This invention relates to a new method wherein Lewis base-borane adducts are utilized as molecular precursors for the formation of both bulk powders, films and coatings of boron nitride.
Interest in the development of ceramic/ceramic composite materials stems from a desire to improve structural integrity over that of a single ceramic component. For example, ceramic fiber reinforced ceramics are known to exhibit increased strength and toughness due to a lessening of crack propagation. Pipes, B.R., McCullough, R.L., Chou, T.W., Scientific American, 1986, 193-203; Bracke, P., Schurmans, H., Vehoest, J., "Inorganic Fibers and Composite Materials", EPO Applied Technology Series Volume 3, Pergamon, New York, 1984. A suitable ceramic fiber coating can enhance the strength of a ceramic fiber/ceramic composite by increasing the interfacial shear strength between the fiber and matrix and thus increase the potential for both debonding and fiber pullout (toughness). Brun, M.K., Singh, R.N., Ceram. Eng. Sci. Proc.. 1987, 8, 636-643; Freeman, G.B., Lackey, W.S., Proc. Annu. Meet. Electron Microsc. Soc. Am., 46th, 1988, 740-741; Brun, M.K., Singh, R.N., Adv. Ceram. Mater., 1988, 5, 506-509; Singh, R.N., Brun, M.K., Adv. Ceram. Mater., 1988, 3, 235-7. Another benefit of fiber coatings is that they may serve as a diffusion barrier between fibers and matrix materials and, thus, inhibit chemical reactions between these materials at high temperatures. Boron nitride (BN) is a non-oxide ceramic which because of its excellent strength and chemical resistance is an attractive prospect as a ceramic coating for fibers in ceramic fiber/ceramic composites.
Previous methods for the formation of coatings or thin films of BN have generally relied on the use of vapor deposition (CVD) techniques, employing mixtures of NH.sub.3 and volatile borane species such as BCl.sub.3, B.sub.2 H.sub.6 and B.sub.3 N.sub.3 H.sub.6. Gmelin Handbuch der Anorganischen Chemie, Boron Compounds, 1980, Third Supplement, Vol. 3, Sec 4 and references therein. For example, conventional CVD techniques have been used for the preparation of thin films of BN from a BCl.sub.3 --NH.sub.3 --H.sub.2 mixture at 1000.degree.-1400.degree. C., while plasma assisted CVD of a B.sub.2 H.sub.6 --NH.sub.3 --H.sub.2 mixture results in a deposition of a thin layer of BN in the temperature range of 400.degree.-700.degree. C. Lowden, R.A., Besmann, T.M., Stinton, D.P., Ceram. Bull. 1988, 67, 350-355. Although the CVD technique offers an effective pathway for depositing a uniform layer of a ceramic on a variety of substrates, these procedures are often time consuming and costly.
An alternative method for generating BN coatings could employ a coatable, non-volatile chemical precursor which could be thermally decomposed to BN on a desired substrate. Indeed, several boron based polymer systems displaying this set of properties have been developed as potential precursors to BN coatings, although such applications have not been reported. Narula, C.K., Schaeffer, R., Paine, R.T., J. Am. Cer. Soc. 1987, 109, 5556-5557; Narula, C.K., Paine, R.T., Schaeffer, R., Polymer Prep. (Am. Chem. Soc. Div. Polym. Chem.) 1987, 28, 454; Narula, C.K., Paine, R.T., Schaeffer, R. in Better Ceramics Through Chemistry II, Brinker, C.J., Clark, D.E., Ulrich, D.R. Eds, MRS Symposium Proceedings 73, Materials Research Society:Pittsburgh Pa., 1986, 363-388; Narula, C.K., Paine, R.T., Schaeffer, R., in Inorganic and Organometallic Polymers, Zeldin, M., Wynne, K.J., Allcock, H.S. Eds., ACS Symposium Series 360, American Chemical Society: Washington, D.C. 1988, 378-384; Paciorek, K.J.L., Harris, D.H., Krone-Schmidt, W., Kratzer, R.H., Technical Report No. 4, Ultrasystems Defense and Space Inc., Irvine, Calif. 1978; Paciorek, K.J.L., Krone-Schmidt, W., Harris, D.H., Kratzer, R.H., Wynne, K.J. in Inorganic and Organometallic Polymers, Zeldin, M., Wynne, K.S., Allcock, H.S., Eds., ACS Symposium Series 360, American Chemical Society: Washington, D.C. 1988, 27, 3271; Rees, W.S., Seyferth, D., presented at the 194th National Meeting of the American Chemical Society, New Orleans, La., Sep. 1987, Paper INOR 446; Rees, W.S., Jr., Seyferth, D., J. Am. Ceram. Soc., 1988, 71, C194-C196; Mirabelli, M.G.L., Sneddon, L.G., Inorg. Chem. 1988, 27, 3721; Mirabelli, M.G.L., Lynch, A.T., Sneddon, L.G., Solid State Ionics, in press.
Simple molecular precursors to BN would also be desirable and would offer a number of advantages over vapor deposition methods including control of stoichiometry, ceramic formation at lower temperatures and higher processability. Perhaps the simplest species containing boron and nitrogen which might be considered as a potential precursor to BN is the Lewis base-borane NH.sub.3 BH.sub.3. The decomposition reactions of NH.sub.3 BH.sub.3 have, in fact, been previously studied by Geanangel who examined reactions of NH.sub.3.BH.sub.3 both in solution and in the solid state. In solution, in aprotic solvents, the compound is found to decompose above 80.degree. C. to various species including cyclotriborazane and borazine. Geanangel, R., Mukherjee, P.J., Wang, J.S., presented at the Boron-USA Conference, Dallas, Tex., April 1988. This result is consistent with earlier observations that, upon standing, solutions of NH.sub.3.BH.sub.3 deposit an insoluble material [BH.sub.2 (NH.sub.3)2].sup.+ BH.sub.4 -, a known precursor of BN cyclics. Shore, S.G., Boddeker, K.W., Inorg. Chem. 1963, 3, 915-916; Niedenzu, K., Dawson, J.W., Boron-Nitrogen Compounds, Academic Press, Inc., New York 1965; Stock, A., Hydrides of Boron and Silicon, Cornell Univ. Press, Ithaca, N.Y. 1933; Sheldon, J.C., Smith, B.C., Quart. Rev. Chem. Soc., 1960, 14, 200; Hu, M.G., Geanangel, R.A., Wendlandt, W.W., Thermochimica Acta. (1978) 23, 249-255. Thermal decomposition of NH.sub.3.BH.sub.3 in the solid-state leads to a variety of products depending on the conditions employed. Heating the compound above its melting point (115.degree.-116.degree. C.) results in partial sublimation and some decomposition leading to the formation of B.sub.2 H.sub.6, B.sub.3 N.sub.3 H.sub.6 and (BNH).sub.x polymeric materials. Liepins, R.A., Geanangel, R.A., Komm, R., Inorg. Chem. 1983, 22, 1684-1686. High temperature pyrolysis (950.degree. C.) of NH.sub.3.BH.sub.3 in a platinum covered glassy carbon crucible has been reported to result in a 65% ceramic yield of BN. Walker, B.E. Jr., Rice, R.W., Becher, P.F., Bender, B.A., Coblenz, W.S., Ceram. Bull, 1983, 62, 916-923.
Despite the fact that bulk pyrolysis of NH.sub.3.BH.sub.3 leads to the formation of BN powder, the applications of this material as a chemical precursor for the generation of BN coatings appear somewhat limited. Indeed, plasma assisted CVD of NH.sub.3.BH.sub.3, even at high energy and lengthy reaction times, produces a material with high hydrogen content, BNH.sub.0.5. Liepins, R., Jorgensen, B., Jahn, R., Geanangel, R.A., Komm, R., Proc. Anu. Int. Conf. Plasma Chem. Technol., 1982, 171-174. Among the inherent drawbacks in the use of NH.sub.3.BH.sub.3 for coating or film formation are its low solubility and stability in most solvent systems, and its volatility under mild thermal conditions. Such problems may, however, be minimized with appropriate tailoring of the base-borane complex. For example, introduction of halogen species to the boron center would result in a reduction of volatility of the base-borane complex. Unfortunately, the compound [NH.sub.3.BHBr.sub.2 ] has never been reported, and based on previous work by Geanangel (Genanangel, R.A., Hu, M.G., Inorg. Chem. 1979, 18, 3297-3301), would be expected to be extremely unstable in solution.